Throughout this application various publications are referenced by arabic numerals within parentheses. Full citations for these publications may be found at the end of the specification immediately preceding the claims. The disclosures of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which this invention pertains. Some of the information set forth herein has been published. See E. Melloni, S. Pontremoli, G. Damiani, P. Viotti, N. Welch, R. A. Rifkind, and P. A. Marks, Vincristine-Resistant Erythroleukemia Cell Line Has Marked Increased Sensitivity To Hexamethylenebisacetamide-Induced Differentiation, Proc. Natl. Acad. Sci USA, 85:3835-3839, June 1988.
Hexamethylene bisacetamide (HMBA)-mediated murine erythroleukemia (MEL) cell terminal differentiation is a multistep process (1,2). Upon culture of MEL cell line 745A-DS19 (DS19) (3) with HMBA (4) there is a latent period of approximately 10 to 12 hours during which commitment to terminal differentiation cannot be detected. Commitment is defined as the capacity to express characteristics of the erythroid differentiated phenotype, including loss of proliferative capacity, despite removal of the inducer (5,6). This early, latent period is followed by a period during which an increasing proportion of the population expresses characteristics of terminal differentiation, including loss of proliferative capacity.
During the latent period the inducer initiates a number of metabolic changes which precede irreversible commitment to differentiation. Among these are alterations in membrane permeability which involve sodium, potassium and calcium flux (7-9), changes in cell volume (10), a transient increase in cyclic AMP concentration (11), a prompt increase in membrane-associated protein kinase C activity (PKC), the appearance in the cytosol of a Ca.sup.2+ and phospholipid-independent form of PKC, presumably generated by proteolytic cleavage of membrane-bound PKC (12), and the modulation in expression of a number of genes, including c-myb, c-myc, c-fos and p53 (13-16). Upon more prolonged culture with HMBA, DS19 cells become irreversibly committed (5,6). Morphological and molecular changes occur which are similar to normal erythroid terminal cell differentiation, including the coordinated expression of genes for .alpha..sup.1 .beta..sup.maj globin, for the heme synthetic enzymes and for erythroid-specific membrane proteins, as well as suppression of DNA replication and rRNA synthesis (1,17,18).
The present invention involves the development of cell lines resistant to an antitumor agent, specifically a MEL cell line derived from DS19 which has been developed is resistant to inhibition of cell growth by vincristine and is designated V3.17. This MEL cell line, V3.17, is unexpectedly markedly more sensitive to HMBA-induced terminal erythroid differentiation.
Another striking characteristic of HMBA induced V3.17 commitment is the absence of the latent period characteristic of induced DS19 differentiation. In addition, the tumor promotor, phorbol-12-myristate-13-acetate (TPA) and the steroid, dexamethasone, both potent inhibitors of HMBA-mediated DS19 cell differentiation (2,19), fail to inhibit differentiation of V3.17 cells.
Moreover, the present invention provides a method for increasing the sensitivity of neoplastic cells to chemical inducers of terminal differentiation, such as HMBA, which involves rendering the cells resistant to an antitumor agent. This method may be used in the treatment of patients having tumors characterized by the proliferation of neoplastic cells.